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Question

A half-cell containing Br and Br^- is connected to a half-cell containing Mg⁺² and Mg to form a voltaic cell. Determine the proper direction for each half-cell reaction to assure a spontaneous, complete cell reaction. Find the value of complete cell electrode potential (E^º).

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Answer 1

Look up the standard reduction potentials for Br and Mg. I find the following:

Mg²⁺ + 2e- ==> Mg(s) Eº = -2.38 V

Br₂(l) + 2e- ==> 2Br^-(aq) Eº = 1.07 V

Accordingly, Br2 will be reduced and Mg(s) will be oxidized in order to have a positive potential.

Br₂ + 2e- ==> 2Br^-

Mg ==> Mg²⁺ + 2e-

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Br₂ + Mg ==> Mg²⁺ + 2Br^- Eº = 1.07 + 2.38 = 3.45 V